"The Challenge" may have made Johnny "Bananas" Devenanzio and Tyler Duckworth rivals ... but Johnny says Tyler was the brother he never had.

Johnny posted an emotional tribute Sunday ... playing Zach Bryan's "Pink Skies" on acoustic guitar as a montage retraced their nearly 20 years of friendship -- from their "Real World: Key West" days to their many battles on "The Challenge."

Waiting for your permission to load the Instagram Media.

Tyler write in the Instagram post ... "You were more than a friend. More than a roommate. You were the brother I never had."

He added ... "Rest easy, Tyler. You'll always be family" ... before closing with "147:3" -- a reference to the Bible verse about healing the brokenhearted and binding their wounds.

Johnny and Tyler entered the MTV world together as roommates on the 2006 season of "The Real World" before repeatedly crossing paths on "The Challenge." After years as competitors, they teamed up and won "The Challenge: Rivals" in 2011.

As TMZ previously reported ... Tyler was found dead Tuesday inside the bathtub of his Grand Forks, North Dakota apartment after a downstairs neighbor reported water leaking through the ceiling. He was 44.

Police said there were no outward signs of trauma and preliminary findings showed no indication of homicide ... his cause of death remains under investigation.